Wind turbine having a reduced radar cross section

ABSTRACT

A wind turbine including a support structure and one or more turbine blades is presented, that incorporates ways for reducing the radar cross section (RCS), wherein the support structure is notionally divided into an upper section in the shadow of the blade sweep area, and a lower section beneath the upper section, wherein the upper section is adapted to have the ways for reducing the RCS, and the lower section does not have the adaptation. The invention makes use of the realisation that the blade masking the tower as it rotates (or the blade being masked by the tower if facing away from a radar), contributes significantly to interference to radar systems, and so localised application of e.g. RAM can give good RCS reduction at a lower cost than treating the whole structure.

This application claims priority to and is a continuation of U.S.application Ser. No. 14/903,403 filed Jan. 7, 2016, which is a nationalphase filing under 35 C.F.R. § 371 of and claims priority to PCT PatentApplication No. PCT/EP2014/066358, filed on Jul. 30, 2014, which claimsthe priority benefit under 35 U.S.C. § 119 of United Kingdom PatentApplication No. 131616.3, filed on Jul. 30, 2013, the contents of eachof which are hereby incorporated in their entireties by reference.

The present invention relates to wind turbines, and to towers used tohold wind turbines. More particularly, it relates to towers andtreatments to such towers, in the form of tiles, coatings, covers, etc.that have an effect on the reflectivity of electromagnetic (EM) wavesused in radars and the like from such towers.

Wind turbines are increasingly being used for power generation, as areplacement for traditional coal and gas power generation, in an attemptto meet national and international carbon emissions targets. Theturbines are generally very large, and act as significant reflectors ofenergy from radar systems, including ATC (Air Traffic Control) radarsand ADR (Air Defence Radar), and weather radars. For this reason newwind farm proposals often face opposition due to the negative effectsthey may have upon such radar systems. The above radars generally employDoppler processing of their return signals, meaning that they look forfrequency changes in the returns caused by target movement. Turbineblades, being moving components, have particular impact upon suchDoppler radars. This can confuse Doppler radars either into thinkingthere are relevant moving targets (e.g. aircraft) present when therearen't, or can mask actual targets from detection. Measures have beentaken to mitigate the effects that wind turbines have upon radarsystems. For example, WO2011/051687 and WO2010/122352 disclosetechniques for minimising the EM reflection from the blades byincorporating EM absorbers, RAM (radiation absorbent material) or thelike within them. Such measures can be very effective, depending uponthe performance of the RAM used.

Reflectivity specifications for wind turbines can be difficult to meet,while keeping costs within reasonable bounds.

According to a first aspect of the present invention there is provided awind turbine comprising at least a support structure and one or moreturbine blades, wherein the support structure is notionally divided intoan upper section and a lower section, the upper section comprisingapproximately that part of the support structure having overlap with theturbine blade's sweep area, and a lower portion being the remaining partof the support structure below it, characterised in that a substantialpart of the upper section is adapted to have a reduced radar crosssection (RCS), and wherein the lower section does not have a substantialpart so adapted.

Note that the overlapped region of the support structure may be thatoverlap region as seen by a remotely located radar, which will typicallyhave a zero or very low angle of elevation in relation to the horizontalplane.

It is known that turbine blades interfere with Doppler radar systems, ashas been noted above. It has hitherto been thought that the supportstructure, although having a significant radar cross section (RCS), hada much reduced effect on Doppler radars as it does not move, and hencedoes not create a Doppler shift. It is also very expensive to coat thetower in RAM. For these reasons, the tower has generally been ignoredwhen considering the effect of the overall turbine on a Doppler radar.However, the present inventors have found that even though the tower isstatic, it can still act in concert with the moving turbine blades toproduce a dynamic return to a radar system within range. This is so evenif the blades of the turbine have been manufactured so as to have areduced RCS using e.g. the techniques proposed in the patent documentsreferenced above.

The effect of the tower on Doppler radars has been found to be primarilydue to the periodic masking of a part of the tower by the turbineblades, as they sweep past the tower, leading to a discontinuity in thephase and/or magnitude of the reflection, which in turn leads to aspreading of the Doppler spectrum. An RCS peak has been found to occurat this point, which dominates the total time averaged RCS from theturbine. This can be much more significant and potentially disruptive toDoppler radar systems than the steady (but often sizeable) reflectionfrom the remaining part of the tower.

The effect has been found to be most pronounced when, in turbines havinga horizontal axis of rotation, the nose of the nacelle holding theblades is pointing in the direction of a radar of interest, i.e. whenthe axis of rotation of the blades is pointing in azimuth towards theradar (and therefore when the azimuthal angle=0 degrees). This isbecause this position leads to a more sudden change in the magnitude ofthe reflection from the tower, as the blade sweeps down past the uppersection of the tower. However, the effect is also significant, but to alesser degree, for small (e.g. less than approx. 45°) azimuth angleseither side of this. There is also an effect when the axis of rotationis pointing at approximately 180° from the radar, i.e. when the nose ofthe nacelle is pointing substantially directly away from the directionof the radar, and for small azimuth angles either side of this.

The adaptation to reduce the RCS may comprise coating the relevant partof the tower with a means for absorbing radiation, such as a RAM, forexample a circuit analogue RAM, or may comprise physical shaping todirect EM radiation away from the direction from which it came, or awayfrom a given direction of interest. A combination of both shaping andRAM coverage may be used.

The parts of the upper section adapted to reduce RCS may, in someembodiments, be the whole length of the upper section, for at least apart of its circumference but positive effects will be obtained if themajority of the length of upper section is so adapted. The adapted partsmay comprise the whole circumference, or may alternatively comprise justa part of the circumference. The adaptation may differ along the lengthof the upper section, so that different sub-lengths have differentadaptations, e.g. differing amounts of RAM coverage. Given the locationof a known radar of interest (i.e. a radar that may be affected by theturbine), then only a portion of the upper section may be adapted toreduce RCS, the portion being that facing towards the radar location,for the reason explained above. A RAM covering may be provided over acircumferential region of between 20° and 90° either side of a pointfacing the radar, with the remainder left uncovered. This thereforeallows significant cost savings compared to covering even just the uppersection of the tower. However, in situations e.g. where the radar may berelocated, or may be located on a moving vehicle, such as an aircraftcoming from any azimuth, then substantially the whole circumference ofthe upper section of the tower may be covered in RAM. Such a completecovering of the upper section may be employed more generally if the costand weight implications are not prohibitive. Alternatively, the uppersection may be divided into sub-sections, with a given sub-sectionhaving a different covering compared to another sub-section. Forexample, different sub-sections may have different circumferentialcoverings, or have a plurality of sectors within the sub-section withdifferent coverings on each.

Embodiments of the invention may have substantially the whole length ofthe upper section adapted to reduce RCS for at least a portion of itscircumference, and may have substantially none of the length of thelower section so adapted. Other embodiments may have substantially thewhole length of the upper section having a RAM covering over at least aportion of its circumference, with substantially none of the length ofthe lower portion having such a RAM covering, but wherein at least aportion of the lower section may be shaped to reduce reflections towardsan associated radar.

The RAM may comprise of RAM tiles, which are glued to the appropriateparts of the tower. The RAM may be a circuit analogue (CA) RAM, whichhas printed resistive tracks of a size, resistivity and separation froman associated ground plane, chosen to be absorptive to wavelengths ofinterest. For most radars, this wavelength would be around 15 cm to 1cm, with Doppler weather radars, for example, typically operating in therange of 1 GHz to 10 GHz (30 cm to 3 cm, maximising the Rayleighback-scatter from the raindrops. The RAM tiles may be similar in regardto their general electrical performance to those disclosed in the abovereferenced patent applications, and of course may be tailored tospecific requirements in known fashion. The tiles preferably have adegree of flexibility to enable them to be attached to the typicallycurved surface of the support structure. However, to add a degree ofstiffness to make them suitable for use in this application, they may behave a substrate layer of glass fibre. This helps to reduce flexing ofthe tiles to acceptable levels, and so reduces cracking of any printedelectrical tracks forming the CA components.

Note that in this application a significant part of the tower, inrelation to the coverage of the RCS reducing treatment, means that theapplication has a significant effect on the RCS for a given wind turbineand radar arrangement. It does not necessarily have to be substantial interms of area of coverage (e.g. it may not, in some embodiments cover amajority of the area of the upper section of the tower), but may be soon some embodiments.

Note also that the upper section of the support structure mayalternatively be defined as being that part of it that goes into shadowof an illuminating radar as a turbine blade sweeps past it. If theilluminating radar is at a different altitude to the support structure,such as at the bottom of a hill on which the support structure islocated, (or vice versa), or is located in an aircraft illuminating thesupport structure from a high altitude, then this alternative definitionmay lead to a small variation in the proportion of the supportstructure's height that is in the upper section. In practice however,the differences are likely to be very small, and thus of littleconsequence to the implementation of the invention.

The support structure comprises primarily a supporting tower, but mayalso include a nacelle that contains the turbine itself, and also themechanical attachment for the turbine blades.

Although the invention is described herein with relation to windturbines having a horizontal axis of rotation (i.e. “HAWTs”, orhorizontal axis wind turbines), it will be appreciated also that theinvention is also applicable to those having a vertical axis of rotationalso (VAWTs, or vertical axis wind turbines) where there is some (butnot total) overlap between the sweep area of the turbine blade and thesupport structure.

According to a second aspect of the present invention there is provideda method of treating a wind turbine support structure comprising:

-   -   i) identifying an upper section of the support structure, the        upper section being that top part having overlap with the sweep        area of a blade of the turbine, with the remainder of the        support structure below it being a lower section    -   ii) identifying a region on the side of the support structure        facing a radar of interest    -   iii) adapting at least a significant part of the circumference        of the upper section of the support structure to reduce its        radar cross section (RCS), with the significant part including a        substantial part of the region identified in (ii); and    -   iv) arranging the lower part of the support structure to not        have a substantial region adapted to reduce its RCS.

The method may be applied to existing wind turbine support structures,or to newly built ones.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. In particular,method aspects may be applied to apparatus aspects, and vice versa.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following Figures, of which:

FIG. 1 diagrammatically illustrates a wind turbine upon whichembodiments of the current invention may be implemented;

FIG. 2 diagrammatically illustrates various blade positions of a turbinewhich lead to the problem addressed by the present invention;

FIG. 3 diagrammatically illustrates a wind turbine having a minimumseparation between its blade and tower that varies along the length ofthe blade;

FIGS. 4-9 diagrammatically illustrate various embodiment of the presentinvention.

FIG. 1 shows a representation of a wind turbine (1) in profile view,comprising a support tower (2), nacelle (3) and turbine blades (4). Thediagram is not shown to scale, and in practice the tower would generallybe longer than shown, in relation to the length of the blades. Theblades (4) rotate about their rotational axis and hence each blade, atthe bottom of its rotation, lines up with the tower (2).

A radar (5) is shown illuminating the turbine (1) with EM radiation,with that part of its illumination of general interest to the currentinvention shown (6). The EM radiation (6) hits the tower (2) and blades(4), and reflects back, and is received by the radar (5) andsubsequently processed in known manner. For most of the rotationalperiod of the turbine the blades are not obscuring any significant areaof the tower. However, when a blade reaches its nadir then it is in linewith the tower, and provides maximum obscuration of part of the tower.The lower blade (4) is shown in this position, and it can be seen to beobscuring the upper part of the tower marked by arrow (7) from the radar(5). Thus the section shown by the arrow (7) is the upper section of thetower, while that part of the tower below it is the lower section. Itcan be seen that the upper section is approximately equal to the lengthof a single blade (4).

FIG. 2 shows in more detail the obscuration process as the bladerotates. FIG. 2a shows a blade (4) rotating about a fulcrum (8), andmoving in the direction shown by the arrow. It is approaching its lowestpoint. Tower (2) is partially obscured, mainly in the upper left part,by the leading edge of the blade, but as the blade sweeps round further,the amount of obscuration will increase quickly.

FIG. 2b shows the blade (4) in its lowest part, and at a point at, orvery close to, maximum obscuration of the tower, dependent upon theexact shape of the blade and the angle from which the blade and towerare being observed. The amount of blade movement between FIGS. 2a and 2bis slight, but the additional obscuration of the tower is significant.Thus the obscuration tends to appear, to a radar, as a sudden transitionwith a discontinuous reflection phase and/or magnitude. In normaloperating conditions the transition occurs sufficiently quickly to showas a large Doppler velocity change, which can lead to the radar giving afalse measurement or detection.

FIG. 2c shows the blade moving away from the lowest point, and startingto reveal the previously obscured tower once more, as the blade'strailing edge moves around. The shape of the blade's trailing edge maydiffer from the leading edge, resulting in a reveal of the tower at adifferent apparent pace to that of its obscuration. This can compoundthe problem this causes to the Doppler radar signal processing.

FIG. 3 shows a type of wind turbine that can create additional RCS peaksdue to the relative shaping of the blades and the support tower. Here,wind turbine (30) has a tower (31) that tapers inwards from its base toits top. A blade (32) is shown positioned at its lowest point. It can beseen that the horizontal separation between the blade and the towervaries along the length of the blade. For example, the separation at theblade tip is shown at A, and the separation at the middle of the bladeshown at B.

Thus, as the blade (32) sweeps past the tower as it moves through itsnadir, as well as the sudden transition effect described with relationto FIG. 2, there is also a more gradual (but still rapid) apparentchange in the average distance of the upper section of the tower aswould be perceived by a Doppler radar. This effect can produce theunwelcome RCS peak effects in such radars.

Prior to this invention, the effects described above were not realisedby those working in the field. Therefore, their solution was to coverall, or a significant part of the whole of the tower in some sort or RAMor equivalent, to prevent reflected radiation from interfering withradar systems. The realisation of the cause has enabled a much lowercost option of highly selective RAM placement just where it has the mostbenefit.

FIGS. 4 to 8 show various embodiments of the invention wherein aselected part of the upper section of the support structure is treatedto reduce reflections back to an associated radar system. In all figuresthe upper section is that part lying above the dotted line (43).

With regards to FIG. 4a , a part of a wind turbine (40) is shown,including the uppermost part of the tower (41) and nacelle and blades(42). Located on the tower is a patch of RAM (44) that stretches fromthe bottom of the upper section to almost the top of the tower. The RAM(44) covers a 90° sector of the tower's circumference, as indicated at(44) in the cross section view at FIG. 4b . The RAM (44) is chosen to beabsorbent to radiation emitted by an associated radar system. Thelocation of the radar system is generally known, particularly if it is afixed, ground based radar. In such circumstances, the RAM is positionedon the tower so that the horizontal centre of the RAM faces the radar'slocation. This gives RAM coverage up to 45° each side of “boresight”direction, to cover instances where the nacelle is not pointing straightat the radar location.

FIG. 5 shows another embodiment of the invention having differenttreatments to reduce the RCS of the upper section. FIG. 5a shows asimilar wind turbine (50) and its support structure (51), having apartial RAM coating (52) on the “front” (i.e. the region facing towardsa known radar position), as in FIG. 4. However, as is evident from thecross-sectional view at FIG. 5b , as well as the front RAM coating (52),there is also a rear coating (53) of similar size, and on an opposingside of the tower (51). This rear coating is present to attenuate radarreturns when the turbine nacelle (42) is facing away from the radar, andhence when there exists a significant possibility for multipathreflections from the blades 54 and the rear of the tower (51).

FIG. 6a shows an embodiment wherein, within the upper region of a windturbine support structure (60), the support structure has two differingRAM covering regimes. A first length of the support structure (60),denoted x has a 90° RAM covering (61) on the front of the tower, as isshown by reference to the cross section of that first length at FIG. 6b. A second length of the support structure (60), denoted y, has a 45°RAM covering (62), as shown in the cross-section view of the secondlength at FIG. 6c . Thus the amount of RAM coating (or indeed any othertechnique used to reduce reflections back to the radar) can be tailoredaccording to the measured or predicted effects of particular parts ofthe support structure. Here, the design reflects the result of suchpredictions or measurements that a lower Doppler return will emanatefrom the second length, and hence a smaller, and hence cheaper, regionof the tower needs to be treated with the RAM coating.

FIG. 7 shows an embodiment of the invention similar to that of FIG. 6,but having a different RAM coverage pattern. FIG. 7a shows the frontview of the turbine unit (70). A first length (x) has a RAM coating (71)covering a 180° span, whereas a second length (y) has a 90° angularcoverage of RAM coating (72). FIG. 7b shows in cross section thecoverage for the first length (x) with RAM coating (71) attached to afront facing part of the tower structure, while FIG. 7c shows in crosssection the coverage for the second length (y).

FIG. 8 shows an embodiment of the invention having both a front and rearcoating of RAM, with differing coverage along the length on the frontface. FIG. 8a shows the front view of a wind turbine (80). On the frontface of the turbine (80) a first length (x) has a RAM coating (81)covering a 90° span, whereas a second length (y) has a 45° angularcoverage of RAM (82) coating the side of the tower. On the rear face ofthe turbine (80) is a strip of RAM (83) of a width that provides a 45°coating running the length of the upper section. FIGS. 8b and 8c showcross sectional views of the tower for the first length (x) and a secondlength (y) respectively, with the sectorial RAM coverage of the lengthsindicated.

FIG. 9a shows an embodiment of the invention wherein a turbine (90) hasa RAM coating (91) covering substantially all of the upper section ofits support structure, on both the front and back. This provides maximumattenuation of radiation that impinges upon the upper section, but alsohas the largest coverage area (and hence cost) and weight. FIG. 9b showsa cross-sectional view of the upper section, where it can be seen thatthe RAM coating (91) covers the whole circumference.

The invention has been described primarily with relation to thecommonplace HAWT version of the wind turbine. The normally skilledperson will appreciate that various modifications and alterations may beapplied to the embodiments described without deviating from the spiritand scope of the invention, including adapting the patterning and areacoverage of any anti-reflection coatings (including RAM), shaping toreduce reflections in any given direction, and use of the invention onVAWT systems.

The invention claimed is:
 1. A wind turbine comprising: a supportstructure; and one or more turbine blades, each of the one or moreturbine blades having a tip at an end thereof, the tip being at a lowestpoint when the turbine blade reaches its nadir upon aligning with thesupport structure, wherein the support structure is notionally divided,by the lowest point of the tip of a turbine blade of the one or moreturbine blades, into an upper section and a lower section, the uppersection including a part of the support structure that overlaps with theturbine blades' sweep area, and the lower section being a remaining partof the support structure below the upper section, wherein a portionspanning from approximately 20° to less than 180° of a circumference ofcertain cross-sections of the upper section is adapted to have a reducedradar cross section (RCS).
 2. The wind turbine as claimed in claim 1,wherein the portion of the upper section is covered with radiationabsorbent material (RAM) and substantially none of the length of thelower section has a reduced cross section (RCS).
 3. The wind turbine asclaimed in claim 2, wherein the RAM is adapted to be absorbent atwavelengths transmitted by an associated radar system.
 4. The windturbine as claimed in claim 1, wherein the portion includes two opposingsections relative to the circumference of the upper section that eachspan approximately 45°.
 5. The wind turbine as claimed in claim 1,wherein, given a predetermined location of a radar in relation to thesupport structure, the portion faces the radar and is covered in RAM. 6.The wind turbine as claimed in claim 5, wherein angular coverage of theportion covered with RAM extends to between approximately 20° to 90°either side of a point on the circumference facing the radar.
 7. Thewind turbine as claimed in claim 6, wherein the angular coverage isapproximately 45° either side of the point facing the radar.
 8. The windturbine as claimed in claim 5, wherein a region on the upper side of thesupport structure opposite to the side facing the radar also has a RAMcoating applied thereto.
 9. The wind turbine as claimed in claim 1,wherein the upper section is itself divided into a plurality ofsub-sections or sub-regions, wherein a given sub-section or sub-regionhas a different covering of RAM as compared to another.
 10. The windturbine as claimed in claim 1, wherein substantially ½ of the uppersection has a RAM covering.
 11. The wind turbine as claimed in claim 1,wherein the RAM comprises a plurality of tiles, each comprising amulti-layered circuit analogue structure.
 12. The wind turbine asclaimed in claim 11, wherein the tiles have a substrate layer comprisingglass fibre.
 13. The wind turbine as claimed in claim 1, wherein theupper section of the support structure is shaped so as to avoidreflection of radio frequency electromagnetic (EM) energy in a directionof interest.
 14. A wind turbine comprising: a support structure; and oneor more turbine blades, each of the one or more turbine blades having atip at an end thereof, the tip being at a lowest point when the turbineblade reaches its nadir upon aligning with the support structure,wherein the support structure is notionally divided, by the lowest pointof the tip of a turbine blade of the one or more turbine blades, into anupper section and a lower section, the upper section including a part ofthe support structure that goes into shadow of an illuminating radar asa turbine blade sweeps past the support structure, and the lower sectionbeing a remaining part of the support structure below the upper section,wherein a portion spanning from approximately 20° to less than 180° of acircumference of certain cross-sections of the upper section is adaptedto have a reduced cross radar section (RCS).
 15. The wind turbine asclaimed in claim 14, wherein substantially none of the length of thelower section has a reduced radar cross section (RCS), and the portionof the upper section is covered with radiation absorbent material (RAM).16. A method of treating a support structure of a wind turbine, the windturbine including a blade, the blade having a tip at an end thereof, thetip being at a lowest point when the blade reaches its nadir uponaligning with the support structure, the support structure beingnotionally divided by the lowest point of the tip into an upper sectionand a lower section, the method comprising: i) identifying the uppersection of the support structure, the upper section being a top parthaving overlap with a sweep area of the blade of the wind turbine, witha remainder of the support structure below it being the lower section;ii) identifying a region on a side of the support structure facing aregion of interest; and iii) adapting at least a portion spanning fromapproximately 20° to less than 180° of a circumference of certaincross-sections of the upper section of the support structure to reduceits radar cross section (RCS), with the portion including a substantialpart of the region defined in (ii).
 17. The method as claimed in claim16, further including: arranging the lower part of the support structuresuch that substantially none of the length thereof is adapted to reduceradar cross section (RCS); and covering the length of the upper sectionwith radiation absorbent material (RAM).